N5
から
Because, Since
昨日、アイスクリームを沢山食べたから、お腹が痛いです。
Since I ate so much ice cream yesterday, my stomach hurts.
Explanation
「から」 also gives a reason: 「危ないですから、触らないでください」 (It's dangerous, so please don't touch). Compared with 「ので」, 「から」 sounds more like the speaker's own judgment and reason, and is more direct. So anything can follow it: a command, a prohibition, an invitation (〜ましょう), an intention, a guess or a request. At the end of a sentence 「〜からです」 alone answers 'why?'. Form: plain form of a verb or い-adjective + から; noun or な-adjective + だから.
Mixed up with
When what follows is a request (〜てください), an invitation (〜ましょう), a prohibition (〜てはいけない) or a command, and the exam asks you to choose between 「から」 and 「ので」, choose 「から」.
When apologising or explaining to a boss or an elder, a bare 「〜から、遅刻しました」 sounds like making excuses and insisting you were right; use 「ので」 instead.